Internal combustion marine drive systems come in several basic types, distinguished by the placement and articulation of the engine and drivetrain components. Differing choices in the layout of these components yield varying results in reliability, performance and ease of maintenance of the systems as a whole.
With an inboard system, a system featured mainly on larger vessels, the engine and almost all of the drivetrain components are placed inside the hull of the vessel towards the bottom, at or below the waterline. The engine and transmission are situated roughly equidistant from the bow, stern, port and starboard sides of the vessel. A propeller shaft extends rearwards from the transmission and tilts slightly downward, exiting the hull behind the inboard engine, ending underneath the bottom and towards the stern of the vessel. The engine of an inboard system can be a marinized automobile type four stroke engine or a purpose-built marine diesel and will typically have its own compartment within the hull. While an inboard engine takes up a good deal of room inside the hull that could otherwise be devoted to interior cabin space, it provides the vessel with excellent balance and a low center of gravity. In addition, the drivetrain used is generally considered the simplest and most efficient method of transferring torque from the engine to the propeller. However, because of the fixed position of the propeller shaft and reliance on a separate stern mounted rudder system, the inboard system is not as maneuverable at low speeds or while in reverse as are other systems.
In contrast an outboard system allows a user to steer by rotating the propeller shaft itself through a large arc. This is made possible by providing the engine, drivetrain and propeller all encased within a single unit externally mounted on the transom of the vessel. Because steering is achieved by rotating this unit as a whole to change the direction of thrust of the propeller, excellent low speed maneuverability is achieved. While the top portion of an outboard system contains the engine components and remains above the waterline, the bottom portion containing the drivetrain and propeller shaft extends beneath the waterline.
The placement of an outboard system on the transom of a vessel tends to make the vessel as a whole heavier at the stern. To minimize the negative effect an outboard system has on the weight balance of a vessel, these systems are designed to be lighter and more compact than an inboard system of comparable power. An outboard system of moderate size can readily be manually removed and replaced on a vessel by a single user. Outboard systems are an attractive option because of their low cost and simplicity.
As a compromise between the inboard system and the outboard system, an inboard/outboard (“I/O”) system combines elements of both aforementioned systems to maximize the utility of each. In an I/O system, as with a true inboard system, the engine is placed inside the hull at or below the waterline and equidistant from the port and starboard sides of the vessel. However the I/O system differs in its placement of the engine towards the stern of the vessel near the transom. An engine driveshaft extends from the engine and exits the vessel through the transom below the waterline. The portion of an I/O system mounted externally on the transom is customarily known as the stern drive, or outdrive, and essentially resembles the lower portion of an outboard system. The stern drive receives the engine driveshaft exiting the vessel through the transom below the waterline and is attached to the transom of the vessel with six large bolts and nuts.
The interior of the stern drive contains a universal joint which enables the rotating shafts housed within the stern drive to turn in a horizontal plane and tilt in a vertical plane while transferring torque from the engine to the propeller shaft. The universal joint is necessary because the stern drive itself must be able to turn and tilt as a unit in order to steer the vessel and to trim the attitude of the vessel, respectively. As is known to those skilled in the art, the stern drive incorporates a gimble unit or other means which allow the lower portion of the unit to be adjusted in the manner described above. See, for example Bland et al U.S. Pat. No. 6,296,535, incorporated herein by reference.
Also provided are a series of gears that allow the rotating shafts inside the stern drive to connect with one another through a series of ninety degree turns. Specifically, these gears allow the engine driveshaft to connect with a vertical shaft, and further allow this vertical shaft to connect with a horizontal propeller shaft. A housing, bellows, and/or other means protect the mechanical components of the stern drive such as the aforementioned gears and universal joint from the corrosive effects of the salt water environment of the stern drive.
The advantages of an I/O system are that a large, fuel efficient automotive type four stroke or marinized diesel engine can be used as with a true inboard. The weight balance of the vessel, while not as good as with a true inboard given the aft placement of the engine, is still better than an outboard system where the weight of the engine rests entirely outside the hull of the vessel. The steering and trimming functionalities of an outboard system are preserved, as is a good deal of interior cabin space in the vessel given the sternward placement of the engine.
Despite their advantages, prior art I/O systems suffer from the notable drawback of susceptibility to failure caused by salt water damage. Because the stern drives in prior art I/O systems are permanently placed below the waterline, their interior mechanical components are vulnerable to damage caused by seawater entering the stern drive. Although bellows are provided to protect the interior mechanical components of the stern drive from the salt water environment in which the stern drive is located, leaks in said bellows do occur necessitating costly repairs for the user. Even if a leak in said bellows does not occur, it is still necessary to replace said bellows on a regular basis, which is also costly for the user.
In addition, routine maintenance tasks such as oil changes and the like can only be performed on the stern drive with the vessel itself removed from the water. Cleaning the exterior housing of the stern drive to remove algae and barnacles can only be performed with the vessel removed from the water or by a trained diver.
As described in the foregoing, a general drawback to I/O systems is the position of the engine near the stern of the vessel. Such a position places the center of gravity of the vessel toward the stern of the vessel. Accordingly, a more unbalanced load distribution is produced by typical I/O systems. The unbalanced load distribution hinders the performance of vessels having I/O systems in comparison to vessels having inboard systems. For example, vessels having an inboard system tend to plane-off better than vessels having outboard systems or I/O systems.
There exists a need for a stern drive for an I/O system that eliminates the problems stated above. It is understood that the present invention relates to a wide range of prior art I/O systems including embodiments not explicitly discussed above. For example, in an alternative embodiment of the prior art I/O system, the stern drive additionally comprises two propellers as well as mechanical means to turn two propellers in opposite directions. Otherwise, this alternative embodiment of the prior art is substantially the same as the system described above. The improved marine inboard/outboard system of the present invention is an improvement over both these embodiments of the prior art.